1. Field of the Invention
This invention generally relates to a process for preparing lower olefin from methanol and/or dimethyl ether, and more particularly to a process for preparing lower olefin from methanol and/or dimethyl ether using an aluminosilicate zeolite-type catalyst in which a calcium-containing compound and a phosphor-containing compound are incorporated.
2. Field of the Invention
Recently, great concern has been shown in the steady supply of petroleum. This is particularly remarkable in Japan because 99% of petroleum consumed in Japan is imported from abroad. In order to overcome such a problem, consideration has been given to the availability of coal, natural gas and the like. In particular, it has been desired to develop an industrial process for synthesizing organic compounds such as olefin, paraffin, an aromatic compound and the like using methanol obtained from methane, CO and the like.
It is widely known in the art that silica-alumina, crystalline aluminosilicate and the like have been conventionally used as a typical catalyst for the conversion of hydrocarbon. Crystalline aluminosilicate possesses a number of pores or fine tunnels of sizes varied depending upon the kind. This causes the aluminosilicate to have shape-selective characteristics of adsorbing, in various kinds of molecules mixed together, only molecules satisfying specific conditions; thus, it is generally called a molecular sieve.
In the 1970s, Mobile Oil Corp. in the United States developed a ZSM-5 type zeolite catalyst as a shape-selective catalyst for preparing hydrocarbon mainly consisting of a high quality gasoline from methanol and/or dimethyl ether. The zeolite has an excellent advantage in that a ratio of SiO.sub.2 /Al.sub.2 O.sub.3 in the composition can be controlled as desired and it has highly improved heat resistance, as compared with conventional zeolite. Also, the zeolite catalyst has satisfactory properties such as a high heat resistance and the like. Further, the catalyst exhibits another advantage capable of converting a main product by the conversion reaction of methanol or dimethyl ether into lower olefin. For example, West-Germany Pat. No. 2935863 discloses that an activation-type zeolite (H-ZSM-5) of which a ratio of SiO.sub.2 /Al.sub.2 O.sub.3 is between 35 and 1600 allows a methanol conversion reaction at a temperature range between 350.degree. C. and 600.degree. C. to produce lower olefin having 2-4 carbon atoms with a yield of up to 70.1 wt.%. Also, the examples of the West-German patent indicate that the optimum ratio of SiO.sub.2 /Al.sub.2 O.sub.3 in the structure of the ZSM- 5 type zeolite catalyst and the optimum reaction temperature of the catalyst are 298.degree.-500.degree. and 550.degree. C., respectively. This clearly reveals that the manufacture of hydrocarbon mainly consisting of lower olefin from methanol and/or dimethyl ether is advantageously carried out at a temperature as high as possible. However, the methanol conversion reaction at such a temperature frequently causes the catalytic action of the ZSM-5 type zeolite catalyst to be rapidly deteriorated at a reaction temperature above 550.degree. C., irrespective of its high heat resistance. Accordingly, in order that lower olefin is manufactured from methanol and/or dimethyl ether used as a starting material at a high temperature above 500.degree. C. with a high yield and without causing the rapid deterioration of catalytic action of the zeolite catalyst for a long period of time, it is highly desired to develop a zeolite catalyst which is capable of substantially decreasing the formation of a coke precursor or BTX (benzene, toluene and xylene) and does not readily cause the deterioration of catalytic activity at a temperature above 550.degree. C.
In view of the foregoing, the inventors made efforts to develop a catalyst free of high-temperature deterioration in the conversion reaction of methanol and/or dimethyl ether at a high temperature range above 500.degree. C. which is advantageous to the formation of lower olefin and, as a result, it was found that microcrystalline pentasil-type zeolite, such as ZSM-5, meets such requirements as described above which is synthesized under conditions of strictly controlling time and temperature for crystallization and a ratio of H.sub.2 O/SiO.sub.2 and has a crystal grain size of the submicron order or below. The zeolite is disclosed in the inventors' earlier Japanese Patent Application Nos. 105537/1984 and 105538/1984 herein incorporated by reference.
Unfortunately, it has been found that even the zeolite catalyst prepared as described above fails to provide a long-term service due to the deterioration of the catalytic action with time.